Polycarbonate moldings have a combination of outstanding properties; high light transmission, high impact strength and a high heat distortion point. The disadvantage of polycarbonate moldings and moldings consisting of all other thermoplastics is their relatively low scratch resistance. Because of the low scratch resistance of these thermoplastics, when they are used in practice many fine scratches which cause optical opaqueness and reduced transparency appear in the surface.
It is known to provide polycarbonate moldings with a polyacrylate coating. This can be effected industrially by coating the polycarbonate moldings with a lacquer based on polyacrylate as described in German Published Patent No. 1,694,273. The coating can also be carried out by pressing polycarbonate sheets together with polyacrylate films under the influence of heat as described in German Published Patent No. 1,953,276.
By coating polycarbonate moldings in this manner, although the system can indeed be provided with a weather-resistant finish, especially if the polyacrylate film applied to the polycarbonate contains a UV absorber, the scratch resistance remains virtually unchanged.
It is also known that polycarbonate moldings can be coated with a scratch-resistant layer based on hydroxylated fluoropolymers crosslinked with methylmelamine. According to one embodiment, as described in German Published Patent No. 1,963,278, this polymer can contain up to 60% by weight of silicon dioxide, relative to the combined weight of silicon dioxide and crosslinked polymer.
The disadvantage of these polycarbonate moldings thus coated is the inadequate stability of these coatings towards weathering. The scratch resistance and the adhesion of the coating decreases after even a very short weathering time. The silicon dioxide-modified coatings are indeed somewhat more weather-resistant than the silicon dioxide-free coatings. Nevertheless, the stability towards weathering of these must also be described as deficient.
As described in German Published Patent No. 1,963,278, it is also known that polyacrylate moldings can be coated with a scratch-resistant covering of a hydroxylated chloropolymer crosslinked with methylmelamine. These coatings on polyacrylate moldings are scratch-resistant and stable towards weathering. The disadvantage of this laminate, however, is the low impact strength.